Project Summary- EPR is a uniquely powerful and widely used tool for solving biomedical problems, from free radical generation to protein structure to in vivo imaging. Too often the EPR signal-to-noise (S/N) is poor for biological samples in commercial X-band (8-12 GHz) reflection type resonators. Previous work has shown that the S/N can be significantly improved by the use of a transmission, or bimodal, type resonator. Such a resonator, called a crossed-loop resonator (CLR), has been developed and used very effectively at frequencies of 3 GHz and below, and prototypes have been constructed at 9.5 GHz (X-band). Using the experience gained in previous CLRs, 2 versions of X band CLRs for 4 mm and 3 mm o.d. sample tubes compatible with commercial EPR spectrometers will be designed and tested. Criteria for success will be ease of operation, reduced dead time after a pulse, and improved signal-to-noise ratio in pulsed EPR. The Specific Aims are: 1. Design an X-band CLR compatible with commercial EPR spectrometers. 2. Build X-band CLRs for 4 mm and 3 mm o.d. sample tubes. 3. Measure the electrical parameters and compare with those predicted. 4. Test trade-offs between the CLRs in research and commercial laboratories. 5. Promote the commercialization of the X-band CLR. Relevance- Study of free radicals in biological systems requires enhanced technology not available in commercial spectrometers. The X-band cross loop resonator will extend the range of studies of free radicals, facilitating new insights into structure of proteins, formation of free radicals in tissues, and the causes of diseases. The major aim of the proposed project is to transfer the technology for this new type resonator from the research lab in which it was invented to a small company that will develop it as a commercial product so that it can be used in many laboratories.